I don't buy the spyware/anti-virus/worm thing. I ran a spyware sweeper, a virus scan, and system cleaner once a week when I used a PC, and it ran like new until I replaced it.

I prefer Mac's because of the way everything meshes together. I love having my contacts synced over my iPhone and address book, along with my calenders and email. I love how Mail, iChat, Address Book, iCal, etc. all seem to work so well together, and I never have to worry about compatibility issues.